A known aggregate application device consists of a container, such as a hopper, for storing the aggregate, a device, such as a discharge roller, for transferring the aggregate from the container to the road surface, and means, such as a feedgate or flow blade, for controlling the amount of aggregate exiting the container. Typically, the feedgate may be adjusted to a particular setting before use so as to apply a particular depth of aggregate to the road.
Two principal means to control the aggregate application rate have evolved in the prior art:    (1) Controlling the extent to which the feedgates are opened (the speed of the discharge roller being fixed).    (2) Controlling the speed of the discharge roller (the extent to which the feedgates are opened being fixed).
Both methods have disadvantages associated therewith. Method (1) is only effective at fast ground speeds (the ground speed being the speed at which the device travels along the ground), and/or high aggregate application rates: at low aggregate application rates and slow ground speeds, the aggregate does not flow through the feedgate gap in a controllable manner. Method (2) is only effective at slow ground speeds: with a fixed opening of the gates, the discharge roller cannot transfer sufficient aggregate with the necessary degree of accuracy at faster ground speeds.
A further disadvantage associated with the known aggregate application device is that it is difficult to ensure a controlled distribution of aggregate across the full discharge width of the device, and therefore across the width of the road. In particular, the known aggregate application devices apply the same amount of aggregate across the width of the device: a problem exists with providing a greater or lesser amount of aggregate at particular sections of the road, eg to give the road a camber, or to fill in ‘ruts’ made by vehicle tyres on old road pavings.
U.S. Pat. No. 5,234,164 to F. K. Hill attempted to alleviate some of the problems outlined above by providing an aggregate application device with a plurality of gates associated with an expandable hopper, together with means for commonly controlling the opening and closing of the gates. Aggregate is transported via a conveyor system from a feed hopper into the expandable hopper, and is dispensed from this expandable hopper onto the road surface through the gates. However, a problem is associated with the device disclosed in this patent, in that the aggregate which it can dispense is limited to that with a substantially constant flow characteristic, and roughly even particle size distribution. Larger particles of aggregate in the material may block the outlet of the gate, which leads to little or no aggregate being applied in this location of the spread width.
UK Patent 2 163 631 to Phoenix Engineering Co. Ltd. describes apparatus for spreading flowable material comprising a main hopper and an extension hopper, both of which are provided with means for discharging the material over substantially the whole length thereof. The extension hopper is movable between a retracted position and an extended position in which it projects beyond one end of the main hopper, thereby enabling variation of the combined width of the hoppers. The device is further provided with means, such as a baffle fixed on the main hopper, for cutting off the communication between the discharging means of the extension hopper and the major part of the interior of the extension hopper over a width corresponding to the longitudinal overlap between the two. Using this apparatus, the width over which the material can be spread can be varied while the apparatus moves along. The device described in this patent is capable of achieving an application rate of up to 135 m/min (440 ft/min).
GB-A-2229105 and GB-A-2021080 disclose aggregate application devices which have a plurality of storage hoppers, each of the hoppers having a gate to adjust the rate of flow of aggregate out of said hopper. Each gate can be set independently of the others at a desired height so as to allow different application rates at different locations across the width of the device. However, the heights are predetermined before the device is used and there is no means to enable the aggregate flow to be varied during operation by variation while in use of the height of the gates.